The field of the invention is chemical and electrical processes for synthesizing metal from a fused bath and the present process is particularly concerned with electrowinning aluminum from a fused bath of cryolite and aluminum compounds.
The state of the art of the electrowinning process begins with U.S. Pat. No. 400,766 and the state of the art of the aluminum reduction cell useful in the present invention may be understood by reference to U.S. Pat. No. 3,400,061 and 4,093,524, the disclosures of which are incorporated herein by reference. Also incorporated by reference herein are U.S. Pat. Nos. 3,028,324; 3,067,124; 3,471,380; 4,333,813; 4,341,611; 4,466,995; 4,466,996; 4,526,911; 4,544,469; 4,560,448; 4,624,766 and European Patent Application 0 021 850 which show the state of the art of protecting cathodes from erosion during the electrowinning of aluminum.
U.S. Pat. Nos. 3,028,324, 3,471,380 and 4,560,448 disclose particular solutions of titanium in molten aluminum.
Most aluminum metal is smelted by being electrowon from alumina, Al.sub.2 O.sub.3 dissolved in a molten salt electrolyte which is mostly cryolite, Na.sub.3 AlF.sub.6 by a process little changed from that described by Hall (U.S. Pat. No. 400,766, 1889). This smelting process is often called the Hall process. The cryolite electrolyte usually also contains several percentage of each of aluminum fluoride, AlF.sub.3 and calcium fluoride, CaF.sub.2. The cryolite electrolyte may also contain several percentage of both magnesium fluoride, MgF.sub.2, and lithium fluoride, LiF. The electrolyte fills most of the bottom part of the cavity of the cell including the vertical gap between the cathodes and anodes. The electrowinning smelting process is carried out at temperatures that may be as low as 920.degree. C. and or as high as 1000.degree. C. The usual operating temperature range is from 950.degree. C. to 975.degree. C. Conventional aluminum smelting cells are well described in THE ENCYCLOPEDIA OF ELECTROCHEMISTRY, Reinhold Publishing Corporation, New York, 1964. These conventional cells are constructed with carbon anodes and in modern cells, carbon block cathodes, called in the industry "cathode blocks." Carbon blocks line the cathode and hold a cathode pool, often called the cathode pad in the industry, containing up to 12 tons of molten aluminum metal that serves electrochemically as the actual cathode. The whole structure, including the carbon cathode blocks, steel electrical current conductors, insulation, and steel pot shell is known in the industry as the " cathode." The anode is geometrically above the cathode by virtue of the fact that cryolite is slightly lighter than aluminum. It floats on top of the molten aluminum metal and washes around the carbon anodes. The anodes are chemically attacked in the electrowinning smelting process and must be individually replaced every two to three weeks. Cathodes must last the expected 3 to 5 year life of the cell.
The pool of molten aluminum is called the cathode or metal pad. In conventional aluminum reduction cells, the metal cathode pool ranges in depth from 5 to 30 centimeters to produce enough hydrostatic pressure to force the molten metal pool into electrical contact with the surface of the carbon cathode blocks. This is necessary because molten aluminum poorly wets the surface of the carbon cathode substrate. Electrical contacts are made with areas of the carbon surface that are momentarily free of electrically insulating materials. At any given moment there are only relatively small areas with good electrical contact between the aluminum pool and the cathode blocks. The remainder of the interface is insulated by a thin layer of molten cryolite, deposits of undissolved alumina ore, and by aluminum carbide, which is a poor electrical conductor. Aluminum carbide readily forms by chemical reaction between molten aluminum metal and carbon of the cathode blocks wherever the two are in contact. Aluminum carbide is somewhat soluble in cryolite electrolyte. It is dissolved away by a layer of cryolite electrolyte, that is normally found between most areas of the metal pool and the carbon cathode despite the hydrostatic pressure exerted by the pool of molten aluminum. Cryolite, not aluminum, prefers to wet carbon and aluminum carbide surfaces. Cryolite is continuously dragged between the aluminum pool and the carbon cathode by motion of the aluminum pool. All areas of the cathode block carbon surface are periodically eaten away by the process of reacting with aluminum metal to form aluminum carbide which is dissolved away by molten cryolite. Wherever aluminum carbide is dissolved away, the carbon cathode blocks may again come briefly into electrical contact with the molten metal of the cathode pool. The carbon surface of the cathode is thus steadily eroded away at rates of from 1 to 5 centimeters per year.
The direct electrical current flowing through current conductors to the cells generates complex magnetic fields that interact with the electrical current flowing in the aluminum pool to cause high velocities in the aluminum pool and to generate waves on its surface. Some waves run from side to side, others from end to end while others rotate around the perimeter of the pot. It is most difficult and expensive to reduce the intensities of the various components of the magnetic field in existing smelters to reduce metal motion and allow the anodes to be moved closer to the cathode. The top surface of the molten aluminum metal cathode pool is covered by standing and moving waves. The tops of the metal waves tend to short circuit the aluminum electrowinning process by making electrical short circuit paths between the anodes and the cathode pool. Such shorting results in losses of 6% to 20% in current efficiency in the aluminum smelting process. Most existing smelters have current efficiencies that range from 78% to 90% out of the possible 98% that can theoretically be obtained. The current efficiency is measured by the total amount of metal actually collected from the cell, divided by the amount that could have been collected if one aluminum atom were produced for every three electrons that flow through the cell.
To reduce electrical shorting and thereby increase current efficiency, the vertical distance between the anodes and the cathode is usually maintained at about 5 centimeters. Cryolite based electrolyte in the gap between the cathode and anodes has an electrical resistivity of about 0.42 ohm-cm and carries a direct electrical current of between 0.7 and 1.5 Amperes/cm.sup.2. The electrical current flowing through the cryolite electrolyte in the gap between the anode and the cathode generates heat, and wastes large amounts of electrical power. The height of the waves on the aluminum pool has been reduced in some of the more recently constructed smelters by computer aided design of the array of electrical conductors that together generate a complex magnetic field pattern in the aluminum pool.
One possible way to prevent the molten aluminum metal from forming surface waves is to remove the metal pool from the cathode surface and to smelt aluminum on raised solid cathode surfaces. An example of this design of aluminum smelting cell is illustrated by Lewis et al (U.S. Pat. No. 3,400,061). Reduction of the vertical gap between the cathodes and anodes to 1 to 2 centimeters can save from 2 to 4 kilo Watt hours of power per kilogram, of aluminum production. This is up to 25% of the power normally required to smelt aluminum by the Hall process. Cell current efficiencies in drained cathode cells are from 5% to 20% greater than in conventional cells. An additional benefit from a drained cathode cell is a decrease of cell polarization of up to about 0.5 volts. The raised cathode surface of a drained cathode cell must be covered by a coating that is readily wetted by the molten aluminum. The coating must not be significantly attacked by either the molten cryolite or molten aluminum during operation of the cell. This coating must last from three to five years to give the cell an economically long life. Carbon may be used as a cathode substrate, but is not useful as the actual cathode surface because it is not spontaneously wetted with molten aluminum and suffers rapid erosion when alternately contacted with aluminum metal and cryolite.
The desire to reduce the electrical power consumption in the smelting of aluminum has resulted in many conceptual designs for aluminum reduction cells and the construction of a few prototype production cells having solid cathode surfaces drained of aluminum metal. For such a cell to smelt alumina efficiently, aluminum metal must easily wet raised solid cathode surfaces so that the electrowon aluminum metal sticks to the cathode surface and drains off into collection areas away from the drained cathode surfaces without being carried off into the cryolite electrolyte as tiny droplets, as is the case when one attempts to smelt aluminum on drained carbon cathode surfaces.
Titanium diboride, TiB.sub.2 has been identified as a material ideally suited to form the solid cathode surface, Ransley (U.S. Pat. No. 3,028,324, 1962). Whenever titanium diboride is mentioned in this application, it must be understood that the borides of Groups IV-B, V-B and VI-B of the periodic table which include the borides of the elements; titanium, zirconium, hafnium, chromium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten and mixtures thereof may be substituted for titanium diboride. Titanium diboride and similar diborides are wetted by aluminum metal, are excellent electrical and thermal conductors and are sparingly soluble in both molten aluminum metal and cryolite based electrolytes.
Some prior United States Patents have attempted to provide aluminum wetted drained cathode surfaces by covering structural cathode carbon blocks with tiles made from titanium and zirconium diborides; Lewis et al (U.S. Pat. No. 3,400,061), Payne (U.S. Pat. No. 4,093,524) and Kaplan (U.S. Pat. No. 4,333,813 and U.S. Pat. No. 4,341,611). Attempts have been made to coat carbon cathode surfaces of drained cathode aluminum reduction cells with smeared coatings composed of titanium diboride mixed with carbon cement; Boxall et al (U.S. Pat. No. 4,544,469; 4,466,692; 4,466,995; 4,466,996; 4,526,911; 4,544,469 and 4,624,766). Attempts have also been made to electroplate titanium diboride coatings on the surface of the carbon cathode substrate prior to producing aluminum metal; Biddulph (European Patent Application 0 021 850).
The various arts found in all previous patents have not yet been commercially successful in providing a durable aluminum wetted cathode surface that is resistant to both molten aluminum metal and cryolite electrolyte. The titanium diboride cathode surface produced by each art suffers from at least one of the following failure mechanisms: the coating material is attacked by aluminum metal or cryolite electrolyte; preformed structural shapes are cracked and broken by rough handling or by stresses caused by uneven thermal expansion during cell start-up; a difference in the coefficient of thermal expansion between the coating and carbon cathode substrate combined with attaching the coating material at room temperature by a cement that becomes brittle at a temperature well below the cell operating temperature results in shear stresses that cause the tiles to disbond; or the carbon cement is chemically attacked by aluminum metal then dissolved by cryolite electrolyte.
Most patented cathode coating systems for aluminum smelting are based on preformed structures containing titanium diboride and carbon cement or brittle mechanical fasteners to attach preformed structures to the carbon cathode substrate. Preformed structures may be pure titanium diboride or mixtures of titanium diboride and bonding materials such as carbon and aluminum nitride. Cements used to attach tiles to the carbon cathode substrate are usually various formulations of carbon cement. The cement joint is subject to fracture while the cell is being heated up to the 970.degree. C. operating temperature, due to stresses resulting from the large difference between the coefficients of thermal expansion between titanium diboride and amorphous and graphitic forms of carbon cathode blocks used to construct aluminum smelting cells. Large shear stresses may develop between titanium diboride preformed structures and the carbon cathode structure because carbon has a lower coefficient of thermal expansion than has titanium diboride. Shear stresses can cause the cement joint to fail and titanium diboride structures to disbond from the cathode blocks, even before the cell starts to operate. These cells can be heated only by slow and careful procedures that properly dry, cure and carbonize the carbon cement to prevent titanium diboride structures from being mechanically damaged by cracking or spalling by stresses resulting from temperature gradients.
If a titanium diboride coating is installed in a cell when it is constructed, special care is required to prevent air from burning carbon cement and titanium diboride during the heating step. Typical means of heating cells for start up are to use oil or gas burners to preheat the cathode surface over a period of 8 to 24 hours to a temperature of about 800.degree. C., while the cathode surface is protected by an inert or chemically reducing material such as a layer of crushed frozen cryolite electrolyte or coke to exclude air. Alternately, the cathode surface can be protected while being heated by the electrical resistance of a layer of coke placed between the anode and the cathode while direct electrical pot line current is passed between the cathode and anodes to slowly heat the cell over a period of a day or two. When the cell reaches a temperature of about 800.degree. C., molten cryolite may be pored into the cell and the process of electrowinning aluminum started. Heat generated by cell electrical resistance and polarization associated with electrowinning aluminum is used to further heat the cell to the equilibrium operating temperature. While the cell smelts aluminum metal at its normal temperature, a balance is maintained between electrical heat generated, heat used by the process and thermal losses. This heat balance is maintained by opening and closing the gap between the anodes and cathodes.
Any cement that holds titanium diboride structures to the carbon cathode substrate that survives cell start up is usually rapidly attacked during cell operation by aluminum and cryolite, by the same processes that attack the carbon cathode surface of a conventional aluminum smelting cell. Aluminum metal also tries to wet the back side of titanium diboride structures causing it to disbond from the carbon cement while converting the carbon cement to aluminum carbide. Cryolite tries to wet the carbon cement and dissolve any aluminum carbide formed from the carbon cement by chemical reaction with the penetrating aluminum metal. Carbon cements react more readily with aluminum to form aluminum carbide than do cathode carbon blocks that are baked at a much higher temperature.
The carbon blocks of the cathode substrate normally undergo from 0.2% to 2% swelling in volume during the first 60 days of cell operation as electroreduced sodium and lithium metals dissolved in the molten aluminum intercalate into the carbon matrix and fluoride salts are absorbed. Any cathode coating system must either swell at the same rate as the carbon blocks or else be able to withstand stresses caused by the expansion of the cathode blocks.
Structural shapes containing titanium diboride and carbon cement that are sintered at temperatures above about 1500.degree. C. are very hard and brittle. These materials are generally too brittle to withstand the incurred stresses and suffer breakage and disbondment from the cathode substrate during carbon cement sintering, cell start up, and normal operation. These structures are also extremely expensive to fabricate and install in the cell.
Another approach to making an aluminum wetted cathode surface is to mix either coarse chunks or finely divided titanium diboride with carbon cement or pitch to form a carbon matrix containing dispersed titanium diboride particles. This wet mixture may be spread onto the cathode surface when building the cell and baked by warming the cell cathode. Alternately this material may be baked into structural shapes at temperatures below 1500.degree. C. and then cemented to the carbon cathode substrate. The resulting material is softer but tougher than titanium diboride preformed structures and generally adheres to the cathode blocks during cell start up. This material however fails rapidly during cell use because molten aluminum penetrates the coating and chemically reacts with the carbon matrix to form aluminum carbide. Aluminum carbide forms first on the top surface and along cracks in the coating. Considerable mechanical expansion occurs during the formation of aluminum carbide since aluminum carbide occupies about four times the volume of the carbon required to form it. As aluminum carbide forms along cracks and as aluminum carbide is dissolved by the cryolite, the coating rapidly disintegrates. Titanium diboride-carbon cathode surface coatings have little resistance to erosion by molten cryolite based electrolytes. These coatings are rapidly attacked by carbon dioxide bubbles that may be periodically swept against its surface. The coatings may fail mechanically by a freeze-thaw mechanism when cryolite freezes on the cathode surface as cold anodes are introduced into the cell every two to three weeks. Both carbon dioxide attack and freeze-thaw damage is more likely when an anode is inadvertently set lower into the cathode cavity than was intended.
No smeared coating or attached preformed structure can be repaired or replaced without freezing the cryolite and shutting the cell down. Cell freezing greatly reduces operating life. Any practical cathode surface coating must be able to withstand mechanical abuse that is normal to cell operation. This includes being poked by steel bars and other tools used to work the cell and make measurements, anodes dropping on the cathode surface, alumina ore deposits that may from time to time fall onto and even freeze to the cathode surface, cryolite electrolyte freezing, occasional burning by carbon dioxide bubbles, electric arcing caused by short circuiting, as well as erosion from cryolite electrolyte. After about 15% of the drained cathode surface area has lost its aluminum wetted coating, the cell looses so much current efficiency that it is no longer economical to operate.